In conventional automatic shift control systems, output speed and torque demand signals are used to control the automatic shift valves. Such shift valves are biased for upshift by an output speed-responsive governor pressure and biased in a downshift direction by a torque demand, i.e., fuel feed, throttle pedal, vacuum or compressor discharge pressure, downshift bias controlled by a torque demand modulated pressure. On a shift from a low or downshift position to a high or upshift position, the shift valve connects mainline pressure to upshift from a low to a high drive ratio and provides the mainline upshift hysteresis force to ensure completion of the upshift and prevent shift cycling or hunting. These automatic shift valves, on an upshift, provide the upshift hysteresis force by having the mainline pressure connected to the high ratio drive acting on an area of the shift valve and by reducing the torque demand bias. The upshift hysteresis force causes the automatic downshift to occur at a considerably lower downshift speed than the upshift speed. This arrangement is advantageous in vehicles or drives having a low weight or load relative to engine power and an engine in which the response and efficiency is good over a rather wide high speed and power range, because it provides less frequent shifting meeting these drive requirements with an efficient and pleasant drive.
In vehicles or drives having a high weight or load relative to engine power, improved automatic shifting is provided by decreasing the torque demand upshift hysteresis bias with increasing torque demand, as shown in Schaefer U.S. Pat. No. 3,587,355, granted June 28, 1971. In such shift controls, the upshift hysteresis force and thus the speed reduction from the upshift speed that will effect a downshift, are reduced with increasing torque demand. Thus as torque demand and power increase, a downshift is made with decreasing speed reduction from the upshift speed to provide more frequent shifting in response to smaller speed changes for more efficiently matching transmission ratio to meet engine output speed and power characteristics for improved engine and transmission efficiency and improved drive characteristics. The mainline hysteresis provides minimum upshift hysteresis force and thus the minimum difference between upshift speed and downshift speed at full throttle to minimize shift cycling.